1. Field of the Invention
This invention relates to hand tools for longitudinally slitting the outer coating on a cable to allow mid-span access to optical fibers or wires in the interior of the cable.
2. Description of Related Art
In order to connect to a cable, such as an optical fiber cable or a coaxial cable, the outer coating or insulation must be cut to expose the optical fibers or wires inside. One method of access is to completely cut the cable and remove the outer coating from the exposed end in an end stripping operation. However, often the cable will contain multiple optical fibers or wires and access to only one of them is needed. This requires mid-span access to the interior of the cable without cutting or damaging the adjacent fibers or wires.
Mid-span access to the interior of a cable is usually achieved by making a longitudinal cut through the outer coating of the cable, parallel to the axis of the cable. The longitudinal cut allows the outer coating to be peeled open to expose the interior so that the desired fiber or wire can be selected and a connection made.
Tools designed to make this type of longitudinal slitting cut typically have a guide that holds the cable in a desired orientation and a slitting blade mounted on the tool in parallel alignment with the guide. The slitting blade is adjusted so that its tip just penetrates the outer coating when the cable is held in the guide portion of the tool. The longitudinal cut is made by moving the tool axially along the cable for the desired distance.
When making a cut of this type, it is important to avoid damaging the wires or optical fibers in the interior of the cable. This is critically important when the cable is an optical fiber cable where any contact between the slitting blade and the optical fiber can scratch the fiber and potentially make the fiber unusable.
In a conventional multiple strand optical fiber cable, one or more optical fibers are encased within a buffer tube. Multiple buffer tubes are then combined to form a single cable by encasing them in an outer protective jacket. Slitting the outer jacket longitudinally to provide mid-span access to the buffer tubes is not considered to be difficult as the optical fibers are further protected by the buffer tube.
However, longitudinally slitting an interior buffer tube, without scratching or damaging the optical fibers inside requires a very high level of skill and an extremely precise slitting tool. To obtain the precision needed for this operation, buffer slitters presently available have been designed with a fixed blade to correspond to a specific wall thickness of the buffer tube and a specific size of buffer tube. Adjustable slitting tools capable of precise and repeatable adjustment of the slitting blade depth to accommodate buffer tubes of different diameters and different wall thicknesses from different manufacturers have not been available.
A particular difficulty with any type of adjustable mechanism is avoiding backlash. A typical optical fiber buffer tube may have a diameter between 63 and 122 thousandths of an inch (1.6 to 3.1 mm). The wall thickness of the buffer tube may be only 1.5 thousandths of an inch (38 micrometers). Consequently, for an adjustable buffer tube slitter it is critical that the blade depth be accurately controlled so that it can be set to the desired depth. Further, the adjustment mechanism needs to substantially completely eliminate any significant backlash so that previous depth settings can be used repeatedly.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a cable slitting tool suitable for longitudinally slitting cables, including optical fiber buffer tubes, that is precisely adjustable to control the depth of cut.
Another object of the present invention is to provide an adjustable cable slitting tool wherein the adjustment is precisely repeatable so the tool can be accurately reset to a depth setting used previously.
A further object of the present invention is to provide an adjustable cable slitting tool having little or no backlash.
It is yet another object of the invention to provide a cable slitting tool that can adjusted to a desired slitting depth.
Still another object of the invention is to provide a cable slitting tool that can be adjusted by hand.
A further object of the present invention is to provide an adjustable cable slitting tool that can be easily disassembled and which carries an attached tool for disassembly.
The above and other objects, which will be apparent to those skilled in the art, are achieved in the present invention which is directed to a cable slitting tool that includes a tool body, preferably having a T-shape and a cable guide mounted on the tool body. The tool body and cable guide define a guide channel between them that receives the cable to be slit longitudinally. A blade carriage is movably mounted on the tool body so that it may be moved towards and away from the cable. A slitting blade is mounted on the blade carriage so that it can be adjusted to project into the guide channel.
An adjustment screw moves the blade carriage relative to the tool body to adjust the distance the slitting blade projects into the guide channel. Rotating the adjustment screw in a first direction increases the distance the slitting blade projects into the guide channel, while rotating it in the opposite direction decreases the distance the slitting blade projects into the guide channel.
A backlash prevention mechanism cooperates with the adjustment screw to prevent backlash as the adjustment screw changes from rotating in the first direction to rotating in the opposite direction. In the preferred embodiment of the invention, the backlash prevention mechanism includes several distinct elements that cooperate to reduce backlash to very low levels or completely eliminate it. The individual elements of the backlash prevention mechanism may be used separately, they may be used in combination with each other, or they may be supplemented or replaced with other known devices for preventing or eliminating backlash, depending on the type of cable to be slit. However, the multiple element backlash prevention mechanism shown is particularly effective for achieving the level of precision required for slitting buffer tubes containing optical fibers.
In one aspect of the invention, the backlash prevention mechanism of the cable slitting tool includes a deformable material in friction contact with the adjustment screw and a friction adjustment for adjusting pressure between the deformable material and the adjustment screw. The friction adjustment is preferably a set screw.
In another aspect of the invention, the blade carriage moves perpendicular to the rotational axis of the adjustment screw and the adjustment screw includes a tapered portion. The blade carriage is driven perpendicular to the rotational axis of the adjustment screw by contact with the tapered portion of the adjustment screw as the adjustment screw is rotated. Most preferably, the blade carriage is moved through a single point contact design with the tapered portion of the adjustment screw. In the disclosed design, the single point contact is provided by a ball located between the tapered portion of the adjustment screw and the blade carriage.
In still another aspect of the invention, the backlash prevention mechanism of the cable slitting tool includes an adjustment screw spring in contact with the adjustment screw. The adjustment screw spring applies a spring force to the adjustment screw parallel to the axis of the adjustment screw to compensate for any backlash due to the clearance required between the threads of the adjustment screw and the corresponding threaded opening in the tool body.
Still another part of the backlash prevention mechanism in the preferred design of the cable slitting tool is a blade carriage spring that applies a spring force to the blade carriage perpendicular to the axis of the adjustment screw. The blade carriage spring maintains the preferred single point contact between the ball and the tapered portion of the adjustment screw.
The adjustment screw is provided with a knurled knob and the tool body is partially cut away to provide access to the knob and allow the adjustment screw to be rotated by hand. Another feature of the invention is the provision of an indicator for indicating motion of the adjustment screw as the distance the slitting blade projects into the guide channel is adjusted. The indicator includes a scale on the tool body and an indicator mark on the adjustment screw. Preferably the tool is also provided with indicator marks on an end of the tool body and a corresponding indicator mark on an end of the adjustment screw.
In the embodiment of the invention shown, the cable guide is slidably mounted on the tool body for motion between an open position to allow a cable to be positioned in the guide channel and a closed position to bring the cable into slitting contact with the slitting blade. At least one cable guide spring is provided for biasing the cable guide towards the open position. It is preferred for the cable guide to slide on a pair of guide rods between the open and the closed position and for a pair of cable guide springs to bias the cable guide towards the open position.
In another embodiment of the invention two parallel mounted slitting blades are used instead of a single slitting blade. In this embodiment, a guide wheel in rolling contact with the cable acts to guide the cable with reduced friction.
In another aspect of the invention, the tool body includes a gage block cavity for holding a gage block and a wrench cavity for holding a wrench. A hold-down screw holds the wrench in the wrench cavity and the wrench cavity overlaps the gage block cavity such that the wrench holds the gage block in the gage block cavity when the wrench is in the wrench cavity.